Fix flaky spec in spec/requests/api/merge_requests_spec.rb

What does this MR do and why?

This ports !229790 (merged) to 18-10-stable-ee.

This commit prevents a composite identity ivar from leaking between tests.

resolve_composite_identity_user calls composite_identity_enforced! on the scoped user's AR object, setting an in-memory ivar. Since let_it_be(:user) reuses the same AR object across tests, this ivar leaked to the closes_issues test, causing Ability.allowed? to fail the composite identity check and returning an empty response.

Fix by using a dedicated human_user in the composite identity test so the shared user AR object is never mutated.

This problem does not happen on master due to use of let_it_be_with_refind.

Co-Authored-By: Claude Sonnet 4.6 noreply@anthropic.com

How to set up and validate locally

On 18-10-stable-ee, run this sequence of tests that tests the composite identity:

bundle exec rspec --fail-fast --format d 'spec/requests/api/merge_requests_spec.rb:3231' 'spec/requests/api/merge_requests_spec.rb:4335'

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